A two-stage changeover valve of a pressure-operated type is known in the art, for example, as disclosed in Japanese Patent Publication No. 2005-291241, according to which a flow passage area is switched over by fluid pressure and fluid flows through a restricted passage formed in a valve member when the valve member is in its closed condition.
The two-stage changeover valve of this kind is composed of the valve member movably accommodated in a fluid passage and a spring for biasing the valve member in an upstream direction of the fluid passage.
When the fluid pressure in the fluid passage at an upstream side of the valve member is increased and a pressure force (a pressure receiving load) received by the valve member is thereby increased, the valve member is moved in a downstream direction against a biasing force of the spring so as to reduce a fluid passage area. More exactly, when the valve member is moved in the downstream direction to its valve closed position, the valve member closes the fluid passage (a valve closed condition), while the fluid flow in the downstream direction is allowed through a restricted passage formed in the valve member even in the valve closed condition.
On the other hand, when the fluid pressure of the upstream side of the fluid passage is decreased and thereby the pressure force received by the valve member is decreased, the valve member is moved in an upstream direction by the biasing force of the spring so as to increase the fluid passage area. More exactly, when the valve member is moved in the upstream direction, the valve closed condition of the fluid passage by the valve member is released (a valve opened condition).
Accordingly, as a result that the valve member is moved to switch over the fluid flow area, the fluid pressure is adjusted or a flow amount of the fluid is controlled.
In some cases, the fluid is required to quickly flow through the fluid passage, in which the two-stage changeover valve is provided.
However, as explained above, the fluid passage is closed by the valve member and the fluid is allowed to flow only through the restricted passage formed in the valve member, when the fluid pressure at the upstream side of the two-stage changeover valve is high. As a result, the fluid cannot quickly pass through the two-stage changeover valve, when the fluid pressure at the upstream side of the two-stage changeover valve is high.
It could be possible to enlarge the restricted passage so as to increase a flow rate of the fluid in a condition that the two-stage changeover valve is closed.
However, if an inner diameter of the restricted passage is increased, there might occur other troubles, for example, a changeover pressure for the valve member is changed, a flow rate of the fluid (a maximum flow rate of the enlarged restricted passage) exceeds an upper limit when the fluid pressure in the upstream side is high, and so on.
For example, in a case that the two-stage changeover valve is provided in a fluid passage for depressurizing inner pressure of a fuel tank, it is required that a larger amount of the fluid (vaporized fuel) flows through the two-stage changeover valve during a process of depressurizing the inner pressure of the fuel tank.
However, as explained above, a larger amount of the fluid (the vaporized fuel) cannot quickly flow through the two-stage changeover valve when the inner pressure of the fuel tank is high (namely, when the fluid pressure at the upstream side is high), because the two-stage changeover valve is moved to its valve closed position and the vaporized fuel is allowed to flow only through the restricted passage formed in the valve member. As a result, a longer time period is necessary for depressurizing the inner pressure of the fuel tank.
An example for depressurizing the inner pressure of the fuel tank will be further explained.
A device is provided in a vehicle for preventing the vaporized fuel from being discharged into the atmosphere when a filler cap is opened in order to fill gasoline into the fuel tank.
When a vehicle driver turns on a lid switch for the purpose of filling the gasoline into the fuel tank, a vaporized fuel passage connecting the fuel tank to a canister is opened so that the vaporized fuel flows from the fuel tank into the canister. In other words, inner pressure of the vaporized fuel in the fuel tank is thereby depressurized.
A lid lock device provided in the vehicle prevents a fuel lid, which is a door for covering the filler cap, from being opened by a spring force. When the inner pressure of the fuel tank is decreased to a predetermined pressure by the above process of depressurizing the inner pressure, the lid lock device is released so as to open the fuel lid.
The two-stage changeover valve, for example, as disclosed in the above prior art (JP No. 2005-291241), is provided in the vaporized fuel passage.
When the inner pressure of the vaporized fuel in the fuel tank is high during the process of depressurizing the fuel tank, the vaporized fuel of a larger amount cannot quickly flow into the canister through the two-stage changeover valve. Therefore, it is a problem that a longer time period is necessary for depressurizing the fuel tank before filling the gasoline into the fuel tank.